The Basics

class

Basic class definitions begin with the
keyword class, followed by a class name,
followed by a pair of curly braces which enclose the definitions
of the properties and methods belonging to the class.

The class name can be any valid label, provided it is not a
PHP reserved word. A valid class
name starts with a letter or underscore, followed by any number of
letters, numbers, or underscores. As a regular expression, it
would be expressed thus:
^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*$.

A class may contain its
own constants, variables
(called "properties"), and functions (called "methods").

The pseudo-variable $this is available when a
method is called from within an object
context. $this is a reference to the calling
object (usually the object to which the method belongs, but
possibly another object, if the method is called
statically from the context
of a secondary object).

class B{ function bar() {// Note: the next line will issue a warning if E_STRICT is enabled.A::foo(); }}

$a = new A();$a->foo();

// Note: the next line will issue a warning if E_STRICT is enabled.A::foo();$b = new B();$b->bar();

// Note: the next line will issue a warning if E_STRICT is enabled.B::bar();?>

The above example will output:

$this is defined (A)
$this is not defined.
$this is defined (B)
$this is not defined.

new

To create an instance of a class, the new keyword must
be used. An object will always be created unless the object has a
constructor defined that throws an
exception on error. Classes
should be defined before instantiation (and in some cases this is a
requirement).

If a string containing the name of a class is used with
new, a new instance of that class will be created. If
the class is in a namespace, its fully qualified name must be used when
doing this.

Example #3 Creating an instance

<?php$instance = new SimpleClass();

// This can also be done with a variable:$className = 'Foo';$instance = new $className(); // Foo()?>

In the class context, it is possible to create a new object by
new self and new parent.

When assigning an already created instance of a class to a new variable, the new variable
will access the same instance as the object that was assigned. This
behaviour is the same when passing instances to a function. A copy
of an already created object can be made by
cloning it.

extends

A class can inherit the methods and properties of another class by
using the keyword extends in the class
declaration. It is not possible to extend multiple classes; a
class can only inherit from one base class.

The inherited methods and properties can be overridden by
redeclaring them with the same name defined in the parent
class. However, if the parent class has defined a method
as final, that method
may not be overridden. It is possible to access the overridden
methods or static properties by referencing them
with parent::.

When overriding methods, the parameter signature should remain the same or
PHP will generate an E_STRICT level error. This does
not apply to the constructor, which allows overriding with different
parameters.

::class

Since PHP 5.5, the class keyword is also used for class
name resolution. You can get a string containing the fully qualified name
of the ClassName class by using
ClassName::class. This is particularly useful with
namespaced classes.

User Contributed Notes 20 notes

I was confused at first about object assignment, because it's not quite the same as normal assignment or assignment by reference. But I think I've figured out what's going on.

First, think of variables in PHP as data slots. Each one is a name that points to a data slot that can hold a value that is one of the basic data types: a number, a string, a boolean, etc. When you create a reference, you are making a second name that points at the same data slot. When you assign one variable to another, you are copying the contents of one data slot to another data slot.

Now, the trick is that object instances are not like the basic data types. They cannot be held in the data slots directly. Instead, an object's "handle" goes in the data slot. This is an identifier that points at one particular instance of an obect. So, the object handle, although not directly visible to the programmer, is one of the basic datatypes.

What makes this tricky is that when you take a variable which holds an object handle, and you assign it to another variable, that other variable gets a copy of the same object handle. This means that both variables can change the state of the same object instance. But they are not references, so if one of the variables is assigned a new value, it does not affect the other variable.

$assignment has a different data slot from $objectVar, but its data slot holds a handle to the same object. This makes it behave in some ways like a reference. If you use the variable $objectVar to change the state of the Object instance, those changes also show up under $assignment, because it is pointing at that same Object instance.

But it is not exactly the same as a reference. If you null out $objectVar, you replace the handle in its data slot with NULL. This means that $reference, which points at the same data slot, will also be NULL. But $assignment, which is a different data slot, will still hold its copy of the handle to the Object instance, so it will not be NULL.

Wouldn't it be great to get the lawn mowed by saying $son->mowLawn()? Assuming the function mowLawn() is defined, and you have a son that doesn't throw errors, the lawn will be mowed.

In the following example; let objects of type Line3D measure their own length in 3-dimensional space. Why should I or PHP have to provide another method from outside this class to calculate length, when the class itself holds all the neccessary data and has the education to make the calculation for itself?

<?php

/* * Point3D.php * * Represents one locaton or position in 3-dimensional space * using an (x, y, z) coordinate system. */class Point3D{ public $x; public $y; public $z; // the x coordinate of this Point.

My absolute favorite thing about OOP is that "good" objects keep themselves in check. I mean really, it's the exact same thing in reality... like, if you hire a plumber to fix your kitchen sink, wouldn't you expect him to figure out the best plan of attack? Wouldn't he dislike the fact that you want to control the whole job? Wouldn't you expect him to not give you additional problems? And for god's sake, it is too much to ask that he cleans up before he leaves?

I say, design your classes well, so they can do their jobs uninterrupted... who like bad news? And, if your classes and objects are well defined, educated, and have all the necessary data to work on (like the examples above do), you won't have to micro-manage the whole program from outside of the class. In other words... create an object, and LET IT RIP!

A PHP Class can be used for several things, but at the most basic level, you'll use classes to "organize and deal with like-minded data". Here's what I mean by "organizing like-minded data". First, start with unorganized data.

Imagination that each function you write only calls the bits of data in that class. Some functions may access all the data, while other functions may only access one piece of data. If each function revolves around the data inside, then you have created a good class.

$ob = new a();a::getFoo(); // output: I am foo $ob->getFoo(); // output: I am foo//a::getBar(); // fatal error: using $this not in object context$ob->getBar(); // output: I am bar // If you keep $bar non static this will work // but if bar was static, then var_dump($this->bar) will output null

// unset($ob);a::setFoo(); // The same effect as if you called $ob->setFoo(); because $foo is static$ob = new a(); // This will have no effects on $foo$ob->getFoo(); // output: I am a new foo

I was doing something similar to this (example is greatly simplified to show logic) and spent a long while trying to work out why I would always get 'A' and never get 'B'. Now, after a few weeks, I have revisited the problem and have worked out why:

The code 'new B' creates a new instance of class B. While class B extends class A, it is a new object and not an extension of the object created by 'new A'

The value of $x is set to 'B' within the object $b, but not in object $a.

If within A::foo(), one was to access $b->x then one would obtain the vale 'B', for example

It should be noted that the new keyword "::class", added in 5.5, WILL NOT PERFORM ANY CHECK on the existence of the targeted class, either in the current namespace or another.

If the targeted class has not an explicitly defined namespace the keyword will JUST PROVIDE a fully qualified name corresponding to:

CURRENT_NAMESPACE + TARGET_CLASS_NAME

This could NOT correspond to any existing class!

In PHP >5.x it is possible to 'USE' a non-existing namespace and/or a non-existing class, as long as the class is not really used in the code.The interpreter will NOT raise any error in these case:

- the USEd namespace doesn't exist<?phpuse This\Is\Just\A\Fake\Namespace\SomewhereExistantClass as NotFoundClass;?>- the class doesn't exist in the defined namespace<?phpuse In\This\Valid\Namespace\Doesnt\Exist\The\GhostClass as NonExistentClass;?>- the class doesn't exist in the current namespace (when not specifying the class with the 'USE' keyword)<?phpuse This\Is\A\Valid\Namespace\GhostClass as NonExistentClass;?>

So, to recap:To avoid tricky side effects, IF you want to USE THE '::CLASS' keyword, ALWAYS REMEMBER TO carefully CHECK IT HAS BEEN DECLARED CORRECTLY in the 'USE' statement (that actually exists in the current or in the defined namespace)

Small subtlety to anyone just starting out: using $this is for referencing an instance of the current class (there are exceptions to this, but on the basic level it is mostly true; please do not downvote this, it's not meant to be a PhD dissertation).

Unfortunately, Arpit's solution creates a new class and leaves the old class inaccessible. If you need access to members of the class you are in you'll be unable to get such access. This can be a huge problem.

By passing $this as a variable by reference, you can access members of the class and even update them. If you don't want to be able to update them, you can simply pass $this to the function but not as a reference.